He works in an industry that does not allow you to email
code home or copy it off of company computers.
If you read through some of the subpoena petitions in the docket[1] such as [2] you'll find that the defense was asking for documentation of conversations indicating that it was a normal thing for him to take work home with him, and his supervisors were aware of it. Since most of the evidence is under seal (as some of it contains trade-secret information), it's hard to say if he found the evidence that he needed. But it does look like it he's at least claiming that it wasn't unusual for him to take his work home with him.
For example:
The defense will demonstrate at trial that Aleynikov
regularly transferred files offsite and to a local
directory on hosts at Goldman’s development network so
that he would be able to continue working productively in
case of a network outage at Goldman to servers hosting
repositories with platform source code. The records
requested in Item 21 will demonstrate that shortly before
Aleynikov left Goldman, there was an outage that made
access to source code unavailable at least for several
hours that impacted developers’ productivity. This
information will aid the defense in demonstrating that
Aleynikov did not have the specific intent necessary to
commit the crimes with which he was been charged.
For example:
[1]: https://ia600209.us.archive.org/9/items/gov.uscourts.nysd.35... [2]: https://ia600209.us.archive.org/9/items/gov.uscourts.nysd.35...